This invention relates to ground fault interrupter (GFI) circuits, and more particularly to an improved GFI circuit that reliably senses undesired ground fault currents of as small as 5 ma and, in response thereto, interrupts the current flow of the electrical distribution system wherein the ground fault current occurs.
A GFI circuit is a device which interrupts a circuit upon the occurrence of a ground fault current of substantially less magnitude than required to activate other types of protective devices. The ground fault current which will activate such a device is also substantially less than the current that normally poses a shock hazard to humans--for example, less than 0.1 ampere. For purposes of this application, the value of ground fault current at which a GFI device is activated will be referred to as the "ground fault trigger current."
In recent years, GFI devices have been required by electrical codes to be activated whenever the ground fault current exceeds 5 ma. In order to reliably sense this relatively small value of current (compared to other values of current that typically flow in an electrical distribution system) a circuit of significant sophistication has been required. Such sophistication is expensive, not only in design costs, but also in manufacturing costs. Moreover, unless a very good quality of circuit components is maintained, such sophistication does not necessarily produce a more reliable circuit.
U.S. Pat. Nos. 3,555,360, 3,558,980, 3,769,548 and 3,859,567, are exemplary of prior art GFI devices. The most common element of these prior art devices is the .-+.differential transformer" which typically includes a toroidal magnetic core upon which a pair of identical primary windings of relatively few turns are wound. These windings are connected in series respectively with the line (or "hot") conductor and the neutral (or "ground") conductor of the electrical distribution system that delivers electrical power from an input power source to a load. These primary windings are wound such that under normal conditions--that is, when no undesirable ground fault connections are present--the net magnetic flux generated is zero. This is because the flux induced by the current flowing to the load through one winding is cancelled by an opposing flux induced by this same current returning from the load through the other winding. If a ground fault connection should be present, however, so that the current flowing to the load through one winding is not equal to the current returning from the load through the other winding, then the net magnetic flux is no longer zero. This non-zero magnetic flux is then used to induce a voltage in a secondary winding of the toroidal core, which secondary winding is connected to the input of an amplifier. The amplifier, in turn, acts through an appropriate control device to open the line conductor and interrupt the current flow therein.
A common problem associated with the use of toroidal magnetic cores as above described is achieving the desired and requisite sensitivity of the GFI device. That is, because the ground fault trigger current is very small, it is typically necessary that the secondary winding have many more turns thereon than each primary winding. This results in a very bulky and costly differential transformer. One solution to this problem has been to induce a bias flux in the magnetic core, such as is described in this inventor's prior patent, U.S. Pat. No. 3,662,218, or in the other patents cited above. Such a bias flux generally serves to overcome core losses and brings the operation of the differential transformer into a higher permeability portion of the B-H curve of the particular toroidal core that is employed. This bias flux is typically introduced into the core by allowing an alternating bias current to flow through the secondary winding, such as is described in all but one of the aforecited patents. (U.S. Pat. No. 3,558,980 teaches introducing the bias current into one of the primary windings rather into the secondary winding.)
A further problem associated with prior art GFI devices is achieving the desired sensitivity of the device given the parameter tolerances of the various parameters, including component values, that make up the device. Such parameters include the coupling coefficient of the windings on the toroidal core, the permeability of the core, the amplifier gain, and so forth. While it has generally been possible to construct prior art GFI devices that are sensitive to the desired ground fault current level, such construction has invariably required the individual adjustment of selected parameters (such as the bias currents, or flux, coupling coefficients, and the like). Such individual adjustments severely hamper the degree to which the device can be economically manufactured in large quantities.
A still further problem associated with all known existing GFI devices is their reliability. There are relatively few homeowners living in homes of newer construction (wherein GFI devices are required by code for all outside, garage, and/or bathroom circuits), or contractors who are required by code to use GFI devices during construction, who have not, at one time or another, been very unhappy with the GFI devices that have been used. Either the device is too sensitive, being falsely tripped (falsely sensing a ground fault current) by the presence of electrical noise or a transient on the line conductor, in which case the GFI device becomes a real nuisance; or the device is not sensitive enough, hazardously allowing a ground fault condition to exist for a dangerously long period of time, in which case the GFI device becomes a real safety hazard. These same problems exist, of course, for the commercial or industrial user of GFI devices as well. Of these problems, the nuisance problem is perhaps the most prevelant because manufacturers of GFI devices typically design their devices to be overly sensitive, therefore erring on the side of safety rather than on the side of inconvenience.